* [management] Return the reason when custom domain validation fails
The validate endpoint returned an empty 202 and ran the check in a
goroutine, so a failed verification only reached the management log.
Run the check in the request and return 204 or 412 with a classified
reason. Resolver errors are not forwarded.
* [management] Address review feedback on custom domain validation
Pass the request context to the lookup and store calls now that the
check runs in the request, compare the self-resolved name
case-insensitively, document the 412 like the other entries, and
check the returned error in the existing tests.
* [management] Match custom domain CNAME targets case-insensitively
DNS names are case-insensitive, so a CNAME pointing at the cluster in
different casing should validate instead of failing as a mismatch.
* [management] Add validation timeout
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Co-authored-by: DevBot NetBird <91386968+netbirddev@users.noreply.github.com>
